Annotation of src/usr.bin/ssh/PROTOCOL, Revision 1.21
1.1 djm 1: This documents OpenSSH's deviations and extensions to the published SSH
2: protocol.
3:
1.2 djm 4: Note that OpenSSH's sftp and sftp-server implement revision 3 of the SSH
5: filexfer protocol described in:
1.1 djm 6:
7: http://www.openssh.com/txt/draft-ietf-secsh-filexfer-02.txt
8:
1.14 djm 9: Newer versions of the draft will not be supported, though some features
10: are individually implemented as extensions described below.
1.1 djm 11:
1.9 djm 12: The protocol used by OpenSSH's ssh-agent is described in the file
13: PROTOCOL.agent
14:
1.16 djm 15: 1. Transport protocol changes
16:
17: 1.1. transport: Protocol 2 MAC algorithm "umac-64@openssh.com"
1.1 djm 18:
19: This is a new transport-layer MAC method using the UMAC algorithm
20: (rfc4418). This method is identical to the "umac-64" method documented
21: in:
22:
23: http://www.openssh.com/txt/draft-miller-secsh-umac-01.txt
24:
1.16 djm 25: 1.2. transport: Protocol 2 compression algorithm "zlib@openssh.com"
1.1 djm 26:
27: This transport-layer compression method uses the zlib compression
28: algorithm (identical to the "zlib" method in rfc4253), but delays the
29: start of compression until after authentication has completed. This
1.2 djm 30: avoids exposing compression code to attacks from unauthenticated users.
1.1 djm 31:
32: The method is documented in:
33:
34: http://www.openssh.com/txt/draft-miller-secsh-compression-delayed-00.txt
35:
1.16 djm 36: 1.3. transport: New public key algorithms "ssh-rsa-cert-v00@openssh.com",
37: "ssh-dsa-cert-v00@openssh.com",
38: "ecdsa-sha2-nistp256-cert-v01@openssh.com",
39: "ecdsa-sha2-nistp384-cert-v01@openssh.com" and
40: "ecdsa-sha2-nistp521-cert-v01@openssh.com"
1.15 djm 41:
1.16 djm 42: OpenSSH introduces new public key algorithms to support certificate
1.15 djm 43: authentication for users and hostkeys. These methods are documented in
44: the file PROTOCOL.certkeys
45:
1.16 djm 46: 1.4. transport: Elliptic Curve cryptography
47:
48: OpenSSH supports ECC key exchange and public key authentication as
49: specified in RFC5656. Only the ecdsa-sha2-nistp256, ecdsa-sha2-nistp384
50: and ecdsa-sha2-nistp521 curves over GF(p) are supported. Elliptic
51: curve points encoded using point compression are NOT accepted or
52: generated.
53:
1.18 markus 54: 1.5 transport: Protocol 2 Encrypt-then-MAC MAC algorithms
55:
56: OpenSSH supports MAC algorithms, whose names contain "-etm", that
57: perform the calculations in a different order to that defined in RFC
58: 4253. These variants use the so-called "encrypt then MAC" ordering,
59: calculating the MAC over the packet ciphertext rather than the
60: plaintext. This ordering closes a security flaw in the SSH transport
61: protocol, where decryption of unauthenticated ciphertext provided a
62: "decryption oracle" that could, in conjunction with cipher flaws, reveal
63: session plaintext.
64:
65: Specifically, the "-etm" MAC algorithms modify the transport protocol
66: to calculate the MAC over the packet ciphertext and to send the packet
67: length unencrypted. This is necessary for the transport to obtain the
68: length of the packet and location of the MAC tag so that it may be
69: verified without decrypting unauthenticated data.
70:
71: As such, the MAC covers:
72:
1.19 djm 73: mac = MAC(key, sequence_number || packet_length || encrypted_packet)
1.18 markus 74:
1.19 djm 75: where "packet_length" is encoded as a uint32 and "encrypted_packet"
76: contains:
1.18 markus 77:
78: byte padding_length
79: byte[n1] payload; n1 = packet_length - padding_length - 1
80: byte[n2] random padding; n2 = padding_length
81:
1.20 markus 82: 1.6 transport: AES-GCM
83:
84: OpenSSH supports the AES-GCM algorithm as specified in RFC 5647.
85: Because of problems with the specification of the key exchange
86: the behaviour of OpenSSH differs from the RFC as follows:
87:
88: AES-GCM is only negotiated as the cipher algorithms
89: "aes128-gcm@openssh.com" or "aes256-gcm@openssh.com" and never as
90: an MAC algorithm. Additionally, if AES-GCM is selected as the cipher
91: the exchanged MAC algorithms are ignored and there doesn't have to be
92: a matching MAC.
93:
1.16 djm 94: 2. Connection protocol changes
95:
96: 2.1. connection: Channel write close extension "eow@openssh.com"
1.1 djm 97:
98: The SSH connection protocol (rfc4254) provides the SSH_MSG_CHANNEL_EOF
99: message to allow an endpoint to signal its peer that it will send no
100: more data over a channel. Unfortunately, there is no symmetric way for
101: an endpoint to request that its peer should cease sending data to it
102: while still keeping the channel open for the endpoint to send data to
103: the peer.
104:
1.2 djm 105: This is desirable, since it saves the transmission of data that would
1.1 djm 106: otherwise need to be discarded and it allows an endpoint to signal local
107: processes of the condition, e.g. by closing the corresponding file
108: descriptor.
109:
110: OpenSSH implements a channel extension message to perform this
1.10 djm 111: signalling: "eow@openssh.com" (End Of Write). This message is sent by
112: an endpoint when the local output of a session channel is closed or
113: experiences a write error. The message is formatted as follows:
1.1 djm 114:
115: byte SSH_MSG_CHANNEL_REQUEST
116: uint32 recipient channel
117: string "eow@openssh.com"
118: boolean FALSE
119:
120: On receiving this message, the peer SHOULD cease sending data of
121: the channel and MAY signal the process from which the channel data
122: originates (e.g. by closing its read file descriptor).
123:
124: As with the symmetric SSH_MSG_CHANNEL_EOF message, the channel does
125: remain open after a "eow@openssh.com" has been sent and more data may
126: still be sent in the other direction. This message does not consume
127: window space and may be sent even if no window space is available.
128:
1.12 djm 129: NB. due to certain broken SSH implementations aborting upon receipt
130: of this message (in contravention of RFC4254 section 5.4), this
131: message is only sent to OpenSSH peers (identified by banner).
132: Other SSH implementations may be whitelisted to receive this message
133: upon request.
134:
1.16 djm 135: 2.2. connection: disallow additional sessions extension
136: "no-more-sessions@openssh.com"
1.6 djm 137:
138: Most SSH connections will only ever request a single session, but a
139: attacker may abuse a running ssh client to surreptitiously open
140: additional sessions under their control. OpenSSH provides a global
141: request "no-more-sessions@openssh.com" to mitigate this attack.
142:
143: When an OpenSSH client expects that it will never open another session
144: (i.e. it has been started with connection multiplexing disabled), it
145: will send the following global request:
146:
147: byte SSH_MSG_GLOBAL_REQUEST
148: string "no-more-sessions@openssh.com"
149: char want-reply
150:
151: On receipt of such a message, an OpenSSH server will refuse to open
152: future channels of type "session" and instead immediately abort the
153: connection.
154:
155: Note that this is not a general defence against compromised clients
156: (that is impossible), but it thwarts a simple attack.
157:
1.12 djm 158: NB. due to certain broken SSH implementations aborting upon receipt
159: of this message, the no-more-sessions request is only sent to OpenSSH
160: servers (identified by banner). Other SSH implementations may be
161: whitelisted to receive this message upon request.
162:
1.16 djm 163: 2.3. connection: Tunnel forward extension "tun@openssh.com"
1.7 djm 164:
1.8 djm 165: OpenSSH supports layer 2 and layer 3 tunnelling via the "tun@openssh.com"
1.7 djm 166: channel type. This channel type supports forwarding of network packets
1.8 djm 167: with datagram boundaries intact between endpoints equipped with
1.7 djm 168: interfaces like the BSD tun(4) device. Tunnel forwarding channels are
169: requested by the client with the following packet:
170:
171: byte SSH_MSG_CHANNEL_OPEN
172: string "tun@openssh.com"
173: uint32 sender channel
174: uint32 initial window size
175: uint32 maximum packet size
176: uint32 tunnel mode
177: uint32 remote unit number
178:
179: The "tunnel mode" parameter specifies whether the tunnel should forward
180: layer 2 frames or layer 3 packets. It may take one of the following values:
181:
182: SSH_TUNMODE_POINTOPOINT 1 /* layer 3 packets */
183: SSH_TUNMODE_ETHERNET 2 /* layer 2 frames */
184:
185: The "tunnel unit number" specifies the remote interface number, or may
1.13 djm 186: be 0x7fffffff to allow the server to automatically chose an interface. A
187: server that is not willing to open a client-specified unit should refuse
188: the request with a SSH_MSG_CHANNEL_OPEN_FAILURE error. On successful
189: open, the server should reply with SSH_MSG_CHANNEL_OPEN_SUCCESS.
1.7 djm 190:
191: Once established the client and server may exchange packet or frames
192: over the tunnel channel by encapsulating them in SSH protocol strings
193: and sending them as channel data. This ensures that packet boundaries
194: are kept intact. Specifically, packets are transmitted using normal
195: SSH_MSG_CHANNEL_DATA packets:
196:
197: byte SSH_MSG_CHANNEL_DATA
198: uint32 recipient channel
199: string data
200:
201: The contents of the "data" field for layer 3 packets is:
202:
203: uint32 packet length
204: uint32 address family
205: byte[packet length - 4] packet data
206:
207: The "address family" field identifies the type of packet in the message.
208: It may be one of:
209:
210: SSH_TUN_AF_INET 2 /* IPv4 */
211: SSH_TUN_AF_INET6 24 /* IPv6 */
212:
213: The "packet data" field consists of the IPv4/IPv6 datagram itself
214: without any link layer header.
215:
1.13 djm 216: The contents of the "data" field for layer 2 packets is:
1.7 djm 217:
218: uint32 packet length
219: byte[packet length] frame
220:
1.8 djm 221: The "frame" field contains an IEEE 802.3 Ethernet frame, including
1.7 djm 222: header.
223:
1.16 djm 224: 3. SFTP protocol changes
225:
226: 3.1. sftp: Reversal of arguments to SSH_FXP_SYMLINK
1.1 djm 227:
228: When OpenSSH's sftp-server was implemented, the order of the arguments
1.8 djm 229: to the SSH_FXP_SYMLINK method was inadvertently reversed. Unfortunately,
1.1 djm 230: the reversal was not noticed until the server was widely deployed. Since
231: fixing this to follow the specification would cause incompatibility, the
232: current order was retained. For correct operation, clients should send
233: SSH_FXP_SYMLINK as follows:
234:
235: uint32 id
236: string targetpath
237: string linkpath
238:
1.16 djm 239: 3.2. sftp: Server extension announcement in SSH_FXP_VERSION
1.1 djm 240:
241: OpenSSH's sftp-server lists the extensions it supports using the
242: standard extension announcement mechanism in the SSH_FXP_VERSION server
243: hello packet:
244:
245: uint32 3 /* protocol version */
246: string ext1-name
247: string ext1-version
248: string ext2-name
249: string ext2-version
250: ...
251: string extN-name
252: string extN-version
253:
254: Each extension reports its integer version number as an ASCII encoded
255: string, e.g. "1". The version will be incremented if the extension is
256: ever changed in an incompatible way. The server MAY advertise the same
257: extension with multiple versions (though this is unlikely). Clients MUST
1.8 djm 258: check the version number before attempting to use the extension.
1.1 djm 259:
1.16 djm 260: 3.3. sftp: Extension request "posix-rename@openssh.com"
1.1 djm 261:
262: This operation provides a rename operation with POSIX semantics, which
263: are different to those provided by the standard SSH_FXP_RENAME in
264: draft-ietf-secsh-filexfer-02.txt. This request is implemented as a
265: SSH_FXP_EXTENDED request with the following format:
266:
267: uint32 id
268: string "posix-rename@openssh.com"
269: string oldpath
270: string newpath
271:
272: On receiving this request the server will perform the POSIX operation
273: rename(oldpath, newpath) and will respond with a SSH_FXP_STATUS message.
274: This extension is advertised in the SSH_FXP_VERSION hello with version
275: "1".
276:
1.16 djm 277: 3.4. sftp: Extension requests "statvfs@openssh.com" and
1.2 djm 278: "fstatvfs@openssh.com"
1.1 djm 279:
280: These requests correspond to the statvfs and fstatvfs POSIX system
281: interfaces. The "statvfs@openssh.com" request operates on an explicit
282: pathname, and is formatted as follows:
283:
284: uint32 id
285: string "statvfs@openssh.com"
286: string path
287:
1.8 djm 288: The "fstatvfs@openssh.com" operates on an open file handle:
1.1 djm 289:
290: uint32 id
1.2 djm 291: string "fstatvfs@openssh.com"
1.1 djm 292: string handle
293:
294: These requests return a SSH_FXP_STATUS reply on failure. On success they
295: return the following SSH_FXP_EXTENDED_REPLY reply:
296:
297: uint32 id
1.4 dtucker 298: uint64 f_bsize /* file system block size */
299: uint64 f_frsize /* fundamental fs block size */
1.1 djm 300: uint64 f_blocks /* number of blocks (unit f_frsize) */
301: uint64 f_bfree /* free blocks in file system */
302: uint64 f_bavail /* free blocks for non-root */
303: uint64 f_files /* total file inodes */
304: uint64 f_ffree /* free file inodes */
305: uint64 f_favail /* free file inodes for to non-root */
1.3 djm 306: uint64 f_fsid /* file system id */
1.4 dtucker 307: uint64 f_flag /* bit mask of f_flag values */
308: uint64 f_namemax /* maximum filename length */
1.1 djm 309:
310: The values of the f_flag bitmask are as follows:
311:
312: #define SSH_FXE_STATVFS_ST_RDONLY 0x1 /* read-only */
313: #define SSH_FXE_STATVFS_ST_NOSUID 0x2 /* no setuid */
314:
1.11 djm 315: Both the "statvfs@openssh.com" and "fstatvfs@openssh.com" extensions are
316: advertised in the SSH_FXP_VERSION hello with version "2".
1.3 djm 317:
1.17 djm 318: 10. sftp: Extension request "hardlink@openssh.com"
319:
320: This request is for creating a hard link to a regular file. This
321: request is implemented as a SSH_FXP_EXTENDED request with the
322: following format:
323:
324: uint32 id
325: string "hardlink@openssh.com"
326: string oldpath
327: string newpath
328:
329: On receiving this request the server will perform the operation
330: link(oldpath, newpath) and will respond with a SSH_FXP_STATUS message.
331: This extension is advertised in the SSH_FXP_VERSION hello with version
332: "1".
333:
1.21 ! djm 334: 10. sftp: Extension request "fsync@openssh.com"
! 335:
! 336: This request asks the server to call fsync(2) on an open file handle.
! 337:
! 338: uint32 id
! 339: string "fsync@openssh.com"
! 340: string handle
! 341:
! 342: One receiving this request, a server will call fsync(handle_fd) and will
! 343: respond with a SSH_FXP_STATUS message.
! 344:
! 345: This extension is advertised in the SSH_FXP_VERSION hello with version
! 346: "1".
! 347:
! 348: $OpenBSD: PROTOCOL,v 1.20 2013/01/08 18:49:04 markus Exp $
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